Virion assembly defect of polyomavirus hr-t mutants: underphosphorylation of major capsid protein VP1 before viral DNA encapsidation.

The major capsid protein of polyomavirus, VP1, was separated into at least four subspecies by isoelectric focusing. One of these subspecies was selectively extracted from purified virions by mild treatment with sodium dodecyl sulfate, leaving a 140S particle enriched in the other three forms. The two most acidic subspecies were labeled in vivo with [32P]phosphate, and these subspecies are among those identified as being deficient in nontransforming host range (hr-t) mutant virus nonpermissive infection of NIH3T3 cells. Quantitation of VP1 phosphorylation revealed that hr-t mutant virus VP1 is phosphorylated to about 40 to 50% the level of the wild type in NIH3T3 cells, and two-dimensional phosphoamino acid analysis suggested that threonine phosphorylation was affected more than serine phosphorylation. Two results indicate that the VP1 modifications occur before and independent of virus assembly: modified subspecies were detected during wild-type infection within a 2-min pulse-label with [32S]methionine, and VP1 modifications of temperature-sensitive VP1 mutants were the same at both restrictive and permissive temperatures for virus assembly. We conclude that most VP1 modification occurs before viral DNA encapsidation, and that one defect in hr-t mutant virus assembly is in VP1 phosphorylation, primarily affecting threonine.